21209623 has 2 divisors, whose sum is σ = 21209624. Its totient is φ = 21209622.

The previous prime is 21209609. The next prime is 21209633. The reversal of 21209623 is 32690212.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21209623 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21209603) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10604811 + 10604812.

It is an arithmetic number, because the mean of its divisors is an integer number (10604812).

Almost surely, 2^21209623 is an apocalyptic number.

21209623 is a deficient number, since it is larger than the sum of its proper divisors (1).

21209623 is an equidigital number, since it uses as much as digits as its factorization.

21209623 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1296, while the sum is 25.

The square root of 21209623 is about 4605.3906457542. The cubic root of 21209623 is about 276.8073704072.

Adding to 21209623 its reverse (32690212), we get a palindrome (53899835).

The spelling of 21209623 in words is "twenty-one million, two hundred nine thousand, six hundred twenty-three".